The use of mobile broadband (MBB) services is rapidly increasing in all regions of the world as a result of the ongoing transition from cellular telephony to MBB. Mobile data surpassed voice during December 2009 and yearly traffic increases in the order of 200% to 300% have been measured in real networks. This increase is predicted to continue.
Mobile operators now face the challenge of handling this immense traffic increase in their networks. One trend in radio research and regulation is based on the observation that many legacy systems are not using their spectrum very efficiently. While replanning of such legacy systems could free up spectrum for licensed mobile use, significant efforts in research, standardization, and regulation are being spent on finding ways of getting higher spectrum utilization in these bands by means of secondary usage of said spectrum.
A secondary user in this context is a user which is not using the spectrum for its licensed purpose and has obligations to not cause harmful interference to the licensed, or primary, usage. The broadcast TV systems have become the prime target for secondary spectrum usage and regulatory bodies already have rules in place for secondary usage. The main reasons for the interest in the TV spectrum are the stationary and predictability characteristics of the TV transmitters as well as the high value of the TV spectrum bands.
The presence of secondary users implies some sharing of spectrum bands between primary and secondary systems. The sharing of spectrum between two systems is usually grouped into one of the following three categories or approaches: (1) the overlay approach; (2) the underlay approach; or (3) the interweave approach.
The underlay approach uses a very low power per unit of bandwidth such that the interference caused to the primary system is kept below a defined level denoted the interference temperature. This level could be on the order of, or below, the thermal noise.
The interweave approach is the primary-secondary spectrum sharing approach. In this approach the signals of the secondary systems are orthogonalized to the primary signals in the time, frequency, and/or spatial domain(s). This can be achieved by, e.g., letting the secondary systems communicate on time/frequency resources that are unused by primary systems. Another type of interweave is spatial/frequency orthogonalization where channels unused by the primary system at certain locations can be used by secondary systems.
In the overlay approach the secondary system cooperates with the primary system and uses the same spectrum resources for its communication. This can be achieved by, e.g., letting the secondary system forward the primary signals while also transmitting secondary signals on the same communication resources. The approach involves interference management by the secondary system where one possible mechanism is interference cancellation at the secondary receivers in which the primary signal is decoded, reconstructed and subtracted from the received signal which then, ideally, only contains the secondary signal.
Studies show that the channel estimation performance may be an important parameter for the sharing of communication resources. For example, if the channel estimates are poor, the achievable secondary system SNRs will be very low. A straightforward implementation of a shared system transmitter just superimposes the secondary signals on the primary signals without making any modifications of the signal design. This results in the optimal channel estimation performance not being achieved, which limits the performance of the secondary system.